This invention relates to methods for controlling a winder.
In this description, stopping means control of the speed of a winder such that the set target speed is reached at the same instant as the set target length or target diameter in a paper roll being completed or, alternatively, in a paper roll being unwound. If the target speed is zero, the winder stops, but otherwise it continues to run at a new target speed which is slower than the normal running speed.
When a paper or board web that is being completed is wound into a paper roll for customers, paper rolls having a certain diameter or a certain web length are usually needed. Consequently, in the winding process it must be known before the winding is stopped at what stage the winding shall be stopped in order that the roll being completed shall be a roll having a desired diameter or a desired number of meters of web, in which connection in the control of winding it is necessary to know the stopping distance, i.e. the number of meters of web still to be run if stopping is started at the instant in question. Another need of stopping arises when deceleration is performed at a certain location of the roll being unwound (typically a defective area in the roll being unwound) or when there is stopping at the bottom of the roll being unwound before the end of the web is able to be unwound from the roll. The stopping distance is calculated on the basis of an estimated stopping length and the number of remaining meters (target length less actual length) and, in prior art applications, a speed reference is supplied based on this to a rounder, on the basis of which a speed reference is supplied to the drive and, based on this calculation, the set value of speed passed to the rounder is changed upward or downward to control the speed such that winding can be stopped at a desired roll size. This control method known from the state of the art is relatively coarse.
In the prior art arrangements, when stopping takes place according to roll diameter, the paper caliper provided by density measurement has been used in calculation. In the prior art applications, pulse measurements, which are inaccurate and unreliable measurements, have been used in diameter and density measurements.
Moreover, in the prior art methods, taking the delay of the drive into account in the calculation of the estimated stopping length has been somewhat complicated and, in addition, inaccuracies have occurred, in particular if it has been necessary to start deceleration in a situation in which the machine is still accelerating or it has already been in a deceleration phase for some other reason.
In the prior art applications, a “bang-bang” control has been used in controlling deceleration in stopping: either the target value of speed or the target value of deceleration has been switched between two values, and an instantaneous deceleration value has been formed as a mean value. These are coarse and inaccurate means for deceleration control.
With respect to the prior art, reference is made to FI patent No. 80432 (corresponding U.S. Pat. No. 4,631,682), which discloses a method for controlling the operation of a winder, the control system described in the patent providing automatic control of slitter-winder deceleration and stopping at a preset sheet length or a preset roll diameter. This system has used a closed loop control of speed deceleration and automatic compensation for layers removed after a sheet break. This method has been based on the use of two different deceleration values, one of them being used for achieving a desired stopping distance. This prior art method is rather coarse.